1. Field of the Invention
This invention relates to thermoplastic resin compositions suitable for continuous high-speed molding.
2. Prior Art
Speedier production of molded articles from thermoplastic resinous materials is nowadays most essential to cope with ever increasing demand in many industrial sectors. The manufacture for example of monofilaments from olefinic polymers by extension has been sped up from a conventional rate of 100 meters per minute to a modern rate of 150-180 meters. This may be achieved literally by increasing the molding temperature and at the same time the shearing speed, but this would give rise to deterioration by heat of certain thermoplastic resins such as a linear chain low density polyethylene moldable at 200.degree.-250.degree. C. (compared to 140.degree.-160.degree. C. for molding high-pressure low density polyethylene by for example a film extrusion). The problem of thermal deterioration is also associated with certain other types of thermoplastic resins which are so easily plasticizable at low temperature, or which have their intrinsic thermal decomposition temperature approximating their melting and plasticizing temperature, typical examples of such thermoplastic resins being polyamides moldable at 250.degree.-300.degree. C.
Engineering plastics materials moldable at even higher temperature (300.degree.-400.degree. C.) such as liquid crystal polymers including aromatic polyesters, polyphenylenesulfides and the like are likewise regarded problematic in terms of thermal deterioration.
In the case of the manufacture of monofilaments from polyethylene polymer, its deterioration by heat would invite a cross-linking gelation causing draw or extrusion breaks in the monofilament run, and in the case of propylene polymer, thermal deterioration thereof would appear in flow or denier irregularities.
To cope with the above problems of thermal deterioration, it has been proposed to use various oxidation inhibitors such as for example hindered phenol and amines which are effective in capturing free radicals emanating from broken or dissociated molecules at an initial stage of deterioration. More specifically, such radicals are captured by H in the phenolic OH or aminic NH of the respective inhibitor. However, such conventional oxidation inhibitors are susceptible to volatilization at high temperature and hence their ability would decline during ordinary molding of thermoplastic materials and under severe temperature or shearing conditions.
Phosphor-based oxidation inhibitors are also known for use in eliminating discolorization of molded articles such as monofilaments, but are less effective in oxidation inhibiting performance than phenol-based inhibitors.
Olefinic polymers made available with use of highly active Ziegler-type catalysts have residual catalyst components such as halogen compounds which tend to deteriorate the resin, corrode the molds or otherwise produce objectionable gum or white powder during molding of the polymer.